[PATCH 5.10 299/406] fsverity: skip PKCS#7 parser when keyring is empty

Greg Kroah-Hartman
2023-09-17T19:37:32Z

From: Eric Biggers <ebiggers@google.com>

commit 919dc320956ea353a7fb2d84265195ad5ef525ac upstream.

If an fsverity builtin signature is given for a file but the
".fs-verity" keyring is empty, there's no real reason to run the PKCS#7
parser. Skip this to avoid the PKCS#7 attack surface when builtin
signature support is configured into the kernel but is not being used.

This is a hardening improvement, not a fix per se, but I've added
Fixes and Cc stable to get it out to more users.

Fixes: 432434c9f8e1 ("fs-verity: support builtin file signatures")
Cc: stable@vger.kernel.org
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20230820173237.2579-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/verity/signature.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

--- a/fs/verity/signature.c
+++ b/fs/verity/signature.c
@@ -61,6 +61,22 @@ int fsverity_verify_signature(const stru
return -EBADMSG;
}

+ if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
+ /*
+ * The ".fs-verity" keyring is empty, due to builtin signatures
+ * being supported by the kernel but not actually being used.
+ * In this case, verify_pkcs7_signature() would always return an
+ * error, usually ENOKEY. It could also be EBADMSG if the
+ * PKCS#7 is malformed, but that isn't very important to
+ * distinguish. So, just skip to ENOKEY to avoid the attack
+ * surface of the PKCS#7 parser, which would otherwise be
+ * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
+ */
+ fsverity_err(inode,
+ "fs-verity keyring is empty, rejecting signed file!");
+ return -ENOKEY;
+ }
+
d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
if (!d)
return -ENOMEM;